(1)Field of the Invention
The present invention generally relates to spaceborne remote sensing of images, in particular for observing the earth. The invention relates more particularly to a method and a system for three-dimensional location of a target land area by merging images captured by two sensors having different radiometric properties, such as a radar sensor and an optical sensor, embedded on separate satellites.
Two different sensors produce different images of the same land scene being observed. If the images are processed separately, it is generally not possible to locate the pixels in the images accurately. It is necessary to resort to additional information in the form of a digital elevation model DEM, which is interpolated to find the location of each small target area for each image, using the technical features of each sensor.
Processing these images together after corrections of various distortions automatically results in the matching of homologous points in the two images, which are used to estimate an analytical deformation model based on a calculation of the degree of similarity and at least one interpolation for each pair of homologous points.
(2)Description of Related Art
In the article by Jordi Inglada and Alain Giros, “On the Possibility of Automatic Multisensor Image Registration”, IEEE TRANSACTIONS ON GEOSCIENCE AND REMOTE SENSING, vol. 42, no. 10, pages 2104-2120, October 2004, chapter VII presents the estimation of a topography using an optical sensor and a radar sensor on the platforms of two different satellites pointed at a target point. The height h of the target point on the earth, which is considered to be flat, is given by the following formula:h=Δx(tan α1)/((tan α1 tan α2)+1)),wherein α1 and α2 denote the angles of incidence of the target point viewed by the sensors in relation to the earth, and Δx denotes the offset between images of the target point provided by the sensors. The offset Δx is equal to the sum of the distance between the projection of the target point on the earth and the point of intersection of the line of sight of the optical sensor with the earth and the distance between the projection of the target point on the earth and the projection of the target point on the earth perpendicular to the line of sight of the radar sensor, assuming a planar wave front emitted by the radar sensor.
Such an estimation is not accurate enough to locate each small target area in the images three-dimensionally in order to record a three-dimensional topography of the scene. The radar sensor implicitly measures the distance between the phase center of the antenna and the target area. For this reason, the angle of incidence associated with the radar sensor cannot be measured accurately.
If the optical sensor and the radar sensor are embedded on the platform of a single satellite, that is to say α1=α2=α, the previous formula is written as follows:h=Δx(tan α)/((tan α)2+1)), that is to say h=Δx(sin 2α)/2.In practice, the sensors will not be mounted so as to have an identical angle of incidence. There will always be a bias in the location calculation.
Each of these formulae means that the height h of the target point to be measured is proportional to the offset measured between the images of the target point and independent of the elevations of the sensors.
This trigonometric approach to merging data from an optical sensor and a radar sensor is also too simplistic. In order to have a three-dimensional position, other transformations linked to the positions and orientations of the sensors must be taken into account.
The object of the invention is to locate a target land area three-dimensionally with a greater precision by merging acquired data from an optical sensor and a radar sensor, as a function of the positions of the satellites carrying the sensors.
To achieve this object, a method for three-dimensional location of a target land area by means of an optical sensor and a radar sensor embedded respectively on satellites, comprising measuring the positions of the satellites when the sensors point at land areas, is characterized in that it comprises measuring a sight direction of the optical sensor and capturing an image of a land area for each of the measured positions of the optical sensor, measuring the distance between a land area and the radar sensor and capturing an image of the land area for each of the measured positions of the radar sensor, comparing the images captured by the optical sensor and the images captured by the radar sensor, matching the images of land areas captured by the optical and radar sensors and covering common land areas, and determining the distance between the target land area selected from the common land areas and the optical sensor as a function of the positions of the sensors, the distance between the target land area and the radar sensor, and the sight direction of the optical sensor, which are associated with the images captured by the optical and radar sensors covering the target land area.
The distance between the target land area and the optical sensor is thus determined as a function of simple data measured accurately, in particular on board co-located satellites, by an approach involving the merging of two equations relating to the sensors, without making approximations regarding the position of the satellites, allowing the position of the target land area viewed by the sensors to be provided with great accuracy.